When a cataract forms in the human eye, the nucleus of the lens becomes relatively hard and "crystalline," diffusing or scattering light passing through it, and partially or wholly loses its ability to focus. In order to remove the nucleus prior to implanting an artificial lens, the nucleus must be separated from the softer cortex around it inside the lens capsule, and then removed through an opening formed in the capsule.
In the well known phakoemulsification technique, which is becoming the procedure of choice for removing a cataract nucleus, the nucleus is pulverized or abraded by vibration of a "phako" tool. The phako tool has a tip or "needle" (about 3 mm in diameter) which is inserted through an opening in the capsule, into close proximity to the nucleus. High frequency reciprocation of the tip erodes the nucleus into tiny fragments which are then removed by aspiration (flushing) through the the lumen or tube of the tip of the phako tool and removed from the eye. Saline is introduced into the eye between the tip and a sleeve around it. The saline fills the anterior chamber of the eye during the operation to facilitate movement of the cataract particles as they are broken up, aspirated into the needle lumen and flushed out of the eye. One potential danger of the phako technique is that it can very rapidly perforate the posterior lens capsule (which is obscured behind the cataract) if the tip contacts it.